Hi all, I use WD-40 from time to time. It is a moderately good solvent/penetrant, an excellent water displacer and a mediocre lubricant. It does NOT dry up and leave nothing behind! It leaves a light oil film which does not last very long, but long enough for some purposes, such as picking locks. I've never seen it harm any mechanism. Locksmiths often use it, though some still favor powdered graphite. I use it on padlocks that live outdoors. It keeps them from freezing up. I don't see it having much value on bikes, except in cable housings in conjunction with greased cables.
I do a lot of parts cleaning. I use mineral spirits in a sink with a recirculating pump. The other solvent I use is what used to be called Brake Cleaner. The current formulation of most brake cleaners today is a little scary. In order to reduce the use of volatile organic compounds, the formula now includes something which smells awful and gives me headaches. I don't use it. Instead, I use the heptane-based aerosol that used to be called brake cleaner and is now marketed variously as circuit board cleaner, general parts cleaner, etc. It is extremely good at dissolving crud on contact. It leaves no residue, although whatever it dissolves and suspends will still be there if it is allowed to evaporate instead being blown off while still wet, or flushed off with clean solvent or water. I don't like to think of the health implications, though I've read that heptane isn't as nasty as acetone or benzene. I use rubber gloves to keep it off my skin, try to avoid breathing it and would advise any user to do the same.
114 Pearl Street
Mount Vernon, NY 10550